Using a recently proven technique for computing membrane current-voltage relationships in the Neurospora hypha, we will examine in detail the behavior of the primary electrogenic proton pump and the ensemble of secondary transport systems which constitute the membrane "leak" in relation to variations of intracellular and extracellular K ion, H ion, NH4 ion, and Na ions concentrations. The specific objective will be to test, in a quantitative manner, the hypothesis (Harold, Bact. Rev. 36 ion 172) that potassium entry into microorganisms is mediated by a simple K ion - ionophore, while Na ion exit and H ion entry are mediated by an electroneutral exchange system. The degree of coupling between these two systems will be analyzed, along with their relationship to the derepressible NH4 ion (current-carrying) transport mechanism. Electrophysiological and ion-transport measurements will be coordinated, and compared with membrane current estimates obtained from noise analysis. BIBLIOGRAPHIC REFERENCES: Hansen, U.-P. & C.L. Slayman, 1977. Current-voltage relationships for a clearly electrogenic cotransport system. In J.F. Hoffman & S.G. Schultz, eds., "Coupled Transport Phenomena in Cells and Tissues," Raven Press, in press. Slayman, C.L., C.W. Slayman, & U.-P. Hansen, 1977. Current-voltage relationships for the glucose/H ion cotransport system in Neurospora. In M. Thiellier & A. Monnier, eds., "International Workshop on Transmembrane Ionic Exchanges in Plants," C.N.R.S., Paris, in press.